My book just arrived, so I haven't read much yet, but one of the key differences between eating, say 1000 calories in one meal (23:1 IFing) versus under-eating all day (calorie restriction) is that fasting boosts hGH production (see https://intensivedietarymanagement....siology-part-3/) and lipolysis (part-4) and reduces insulin secretion. In my n=1 experiences, undereating slowed my metabolism, whereas IFing (esp with >20 hr fasts) increased my energy and metabolism. A mere look in the mirror shows the difference. On low calorie diets my face looks pale and grey, when IFing it is rosy.